Corrosion-resistant metallic



Sept. 30, 1952 o. E. ANDRUS CORROSION-RESISTANT METALLIC STRUCTURE Original Filed April 6, 1935 llllllllll ll'l l e IIVLIVENTOR Orrin E..Andrus BY q v ATTORNEYS CORROSION-RE SISTANT METALLIC STRUCTURE.

chin e. Andrus, Sh rewood, win, assignor to A. 0. Smith Corpo tion, Milwaukee, Wis., a corporation of New York Original No. 2,267,361, dated December 23, 1941,

Serial'No. 15,146) April 6, 1935. Renewed Nov vember 29, 1937. Application for reissue August 25, 1952, SerialNo. 306,140

8 Claims. (011 204-196) Matter enclosed in heavy brackets I: I appears in the. original patent but forms no part of this 1 peciiication; matter printed in italics indicates the additions made by reissue.

This inventionrelates to corrosionsresistant. metallic structures and has been applied to un-.

derground pipelines such as areused for the transmission 01' gas or oil or water.

The .principal object .of-the. invention is to provide a metallic structure which is resistant to corrosioni'romthe medium contacting therewith Another. .object.oi. the invention .is to provide.

anunderground pipe line resistant to the action or corrosive agencies present in the soil.

Difliculty has been experienced in the past due .to .the corrosion of the pipe and, as a conse-. quence, various expedlents have been proposed for-the purpose of protecting underground pipe lines or other underground structures. In some cases, a pipe line-with bare metallic. surfaces.

buried directly in contact with the soil has been connectedto the. negativeterminal of a source of low voltage direct current, while the positive terminal of the current source has been connected to'an anode .buriedin the .groundat a suitable distanceirom .thepipe. The pipe line.

was thus maintainedat a negativepotential with respect to .its surroundings. The cathodic. protection which was provided in this .way was partially effective in reducing the corrosion of the pipe line, but large. currents. were necessary to .adequately protect a structure" with bare metallic surfaces buried directly in contactwith the soil, and as a consequence of,.the .need for the provision of large currents, the cathodic.

protection of bare pipe lines was expensive in first cost and in maintenance and operation.

Galvanized pipe has also been used, but.because of thelarge expanse andthinness of .the. zinc coatingit is soon consumediand no longer protects the underlying base metal.

It' has also long been the practice to provide pipe with a'coating'or coal tar pitch or asphalt or "similar material to protect the pipe from corrosion. While considerable protection can'be afforded to the pipe by such coatings, it is extremely difficult to provide a coating which is so tree from. defects that no opportunity for localized'corrosion is provided...

In order to protect those-partsof .the pipe which--are-exposed by. defects inthe coating, cathodic :protection has been applied, to: pipes provided with coverings ofpitch or .asphalt :or

similar materials, the theorybeingthat acoverv ing oi this kind will electrically insulate the pipe from. the ground. and that amuch smaller cur-r rent will sumce than isv required-whenbare .me-: tallic surfaces are buried directly-rm contact with;

ing decreases in the course of time and that an...

increasingly larger current is required when the pipe line is kept at the .same negative potential with respect to its surroundings. resistance is in part due to the absorption of moisture and electrolyte from the soil, but whatever the cause it results in an increased current consumption. .For the most part, this increased current is uselessly expanded in leaking through that part of the covering which .is free from gross mechanical defects and which might in itself adequately protect the .pipe line against corrosion even though the coating were low in elec-"' trical resistance. In the limiting case when the electrical resistance of .the coating falls to 3 a1"- most zero, the. current taken from" the current source will be nearlythe same as that requir'e'd to cathodically protect an uncoated pipe line.

An even more serious drawback to the use to! cathodic protection in combination 'with pipes provided with coatings of asphalt, tan-orbitaminous materials, is brought about by the effect which is exerted upon'the coating by the electric current used to cathodically protect those parts of the pipe which are exposed by defe'ctsin thecoating. The electric current passing through the defective spots in the coating protects theunderlying metal from corrosion; but results in the liberation oi'hydrogenu The hydrogen liberated at the cathode as a result of electrolysis develops a hydrogen pressure which increases as the current and the rate of liberation of hydrogen increase. A certain minimum difference of potential is required 'between'the. soil' and. the

pipe in "order that the .pipe may be cathodicaliy;

protected .from corrosion, and even this .mini!. mum. potential difierenceresults in. -currents through. defective spots and conductive areas of sufiicient magnitude .to. build 'up a hydrogen pressure which punctures or loosens coatings of asphalt, tar or bituminousmaterialsa In other-words, the use'of 8.1106186171104013 rent to cathodically protect the. parts of the-wipe The decreased 3 which are exposed by defects in such coatings. results in a gradual enlargement o! the defects which require protection.

The same efiect is also found to some extent.

even where the coating is originally tree irom defects. As the coating has only a finite resistance at best, and one which decreases due to,

the absorption of water and electrolyte from the soil, there is a gradual increase in the current which passes through the covering even where it completely covers the metal. The increased current results in an increased hydrogen pressure which ultimately attains a value sutficient to separate the coating from the metal and to create in the coating a defect where none existed before. Due to this action, there is a gradual deterioration from an initial condition in which the major portion or the pipe is protected from corrosion by the coating and only an insignificant portion by the electric current, towards a final condition in which the coating is ruined by the continued action 01 the current, and such protection as may still be aflorded to the pipe line is furnished almost entirely by the current.

These diiilculties are avoided by the use or this invention. According to the invention, the'pipe or other metallic article is provided with a coating ot vitreous enamel onthose surfaces which areexposed to corrosive agencies. The metal portion of the article is connected to the negative terminal of a low voltage source of unidirectional currentv which may be a battery, a low voltage alternating current transformer associated with a suitable rectifier, a direct current generator,

or other current source capable oi supplying unidirectional current. The positive terminal of the current source is connected to an anode buried in the ground in the vicinity of the pipe or article to .be. protected from corrosion.

Alternatively, the steel base of the pipe line may be connected to an anode which is made 01". zinc or of other metal more active than iron and is buried in the ground on or near the pipe line. No 'othersource or current need be provided (in this case, since if a defect exists in the enamel coating of the pipe, a battery will be formed with the steel or iron of the pipe line acting as the cathode and the zinc as the anode. The pipe line will be protected from corrosion while the anode is being slowly consumed.

The invention is illustrated in the accompanying drawing in which:

Figure 1 is a diagrammatic view showing the application of a source of current to a metallic structure coated withvitreous enamel and sub merged und rground; and

Fig. 2 is a similar view showing the use of an anode of zinc or other metal more active than iron' to protect a steel pipe lineunderground.

. Fig. 3 is a view similar to Figure 1 and shows the use of an anode more active than iron connected to a pipe line by an electrical conductor.

In the drawing the metallic structure 1 has a protective vitreous namel coatin 2 and extends in the ground or other oorroding medium 3.

In Figure 1 the structure 1, which may be a pipe line, is made cathodic by connecting the same to the negative terminal of a battery 4 or other suitable low voltage source of direct current. The positive terminal of battery 4 is connected to an anode 5 submerged in the ground in the vicinity of the structure.

In Figure 2 the structure 1, represented a a pipe line, is made cathodic by securing thereto ansanode 6 of zinc or othermetalmare active structure.

In Figure 3 the structure 1, represented as a p pe line, is made cathodic by connecting thereto anode 7o} zinc or other metal-more active than iron, by an electrical conductor 8.

My tests show that a vitreous enamel coatin applied to. a steel pipe which is-buried underground, maintains a high electrical resistance and is substantially unaffected by water. Furthermore, the bond between the vitreous enamel and the steel pipe is such that the hydrogen pressure developed at detective spots in the enamel is insumcient to puncture the enamel coating or to separate it from the steel base and enlarge the defect. The enamel coating is highly adherent to the steel base, expands or; contracts with the steel base as it changes in dimensions due to changes in temperature or applied forces, and is capable of surviving soil stress effects without damage to its protective properties. The enamel coating alone provides almost complete protection or the pipe from corrosion, and protection which is permanent. Because of the constant high electrical resistance. of the enamel coating, only a small electric current 'i 'ne'eded to supply a long length of a pipe line maintained negative with respect to its surroundings, and to cathodically protect from corrosion those portions of the metal which are exposed by the few defects that are likely to occur in any coating, no matter how carefully it is applied.

Furthermore, the properties of the enamel coating are such that the cathodic protection oi metallic areas exposed by defects in the coating is obtained without enlargement of the defects or damage to the coating. A permanently protected pipe line is provided, and the difllculties of prior methods are substantially eliminated or entirely avoided.

An advantage of the constant high electrical resistance of the enameled covering is that an anode buried in the ground a few hundred feet from the pipe line will permanently protect a much greater length of pipe line from corrosion than would be the case if the coating were of such a nature as to decrease in electrical resistance. Under some conditions, one anode will protect the pipe line for a distance of some five miles on either side. This distance will, of course, depend upon the soil conditions which are encountered, but in any case, the use of enameled pipe will enable the anodes to be spaced more widely along the line than when a line similar in other respects is coated with a composition which decreases in electrical resistance. A further advantage of the permanent character of the properties of a vitreous enameled line in combination with cathodic protection is that the requirements in the way of anodes and current sources to supply the current between the anode and the line can be estimated in advance and will require no extensive additions in the course or time to take care of deterioration in the covering.

In general, the voltage which is used should not be materially greater than is necessary to protect the pipe against corrosion sincethe use oi a needlessly high voltage merely results in a useless expenditure of electrical energy. Experience has shown that the voltage applied between the pipe and the anode shouldbe such that the I claim:

1. A corrosion-resistant metallic structure which comprises a metal base, a coating of vitreous enamel fusedto the metal base, and means to maintain the metal structure electrically negative with respect to its surroundings in order to cathodically protect from corrosion those parts or the metal base which are exposed by defects in the coating.

2. A corrosion-resistant underground metallic structure which comprises a metal base, a coating of vitreous enamel fused to the metal base, a source of low voltage unidirectional electric current, an electrical connection between the metal base and the negative terminal of the source of unidirectional electric current, and an anode which is electrically connected to the positive terminal of the current source and which makes electrical contact with the ground in the vicinity of the underground metallic structure.

3. A corrosion-resistant underground pipe line which comprises steel pipe, a coating of vitreous enamel fused to the steel pipe, and a source of unidirectional electric current with its negative terminal connected to the steel pipe and its positive terminal connected to an anode which makes contact with the ground in the vicinity of the underground pipe line.

4. A corrosion-resistant underground pipe line which comprises steel pipe, a coating of vitreous enamel fused to the steel pipe, and means to maintain the pipe electrically negative with respect to its surroundings in order to cathodically protect from corrosion those part of the pipe which are exposed to the action of corrosive agencies by defects in the enamel coating.

5. A corrosion-resistant underground metallic structure which comprises a metal base, a coating of vitreous enamel applied to said base, said coating being resistant to the action of corrosive agencie present in the soil and of high electrical resistance, and being so firmly bonded to the metal base as to remain undetached therefrom by the hydrogen pressure developed when an electric current passes from the surrounding soil into the metal base through defects in its coating, and means for maintaining the metallic structure electrically negative with respect to its surroundings in order to cathodically protect from corrosion those parts of the structure which are not protected by the coating.

6. A corrosion-resistant underground metallic structure which comprises a metal base. a coating of vitreous enamel fused to the metal base. and an anode of metal more active than the metal base, said anode being buried in the ground in the. vicinity of the underground metallic structure and electrically connected to the metal base by means of an electrical conductor.

'7. A corrosion-resistant pipe line which comprises a steel pipe, a coating of vitreous enamel fused to the steel pipe, and a zinc anode buried in the ground in the vicinity of the pipe line and electrically connected to the steel pipe.

8. A corrosion-resistant metallic structure which comprises a metal base, a coating of vitreous enamel fused to the metal base, and an anode of metal more active than the metal base, said anode being exposed to the medium contacting said base and electrically connected to the metal base by means of an electrical conductor.

ORRIN E. ANDRUS.

REFERENCES CITED The following references are of record in the file of this patent or the original patent:

UNITED STATES PATENTS OTHER REFERENCES Transactions of The American Electrochemical Society,- pre-print 64-13 (1933), p es 146, 147.

"Proceedings of The American Petroleum Institute, vol. 14 (Nov. 1933), section 4, pages 152 and 166. 

